The impact of volcanic ash

Despite its spectacular appearance, it isn't actually the lava that comes out of a volcano that has the most wide-spread and destructive effects. Volcanoes often spew out tonnes of ash, which can travel long distances both in the air and along the ground as deadly pyroclastic flows. Andy Woods shows us how he models these ash flows using a fish tank and some coloured water. But first, he explains what Pliny the younger would be able to see while watching the eruption of Vesuvius in AD 79... 

Andy - At this point, what we're hearing in the description is that the ash emitted from the volcano was rising high into the atmosphere and then spreading out in the atmosphere as a cloud. And as that spread out, it would've formed a very thick cloud, kilometres in vertical extent. And so, no light would be able to come through that so it will get darker and darker. As that cloud spread out radially, the ash falling out would then land on the ground and form a big blanket of ash on the floor.

Chris - Really? Kilometres thick?

Andy - Yeah. So, the typical height of rise of these large plumes is up to 20 - 25 kilometres in these big eruptions. So, the ash is extremely hot. It's got temperatures up to about a thousand degrees when it comes out of the ground. That enormous amount of thermal energy can be converted to potential energy which is the energy you need to lift material high into the atmosphere. In fact, I have a little experiment I brought along to demonstrate how this process works. Behind me, I have a tank which is about a metre high. It's 20 centimetres by 20 centimetres in cross section and it's full of salty water. And the amount of salt in this water decreases as we go up through the tank. So, the water at the bottom of the tank is very salty and as we move to the top of the tank, the water gets much less salty.

Chris - Why is that?

Andy - So, that's the way we set the experiment up in order to model the atmosphere. When you rise up through the atmosphere, the temperature actually gets warmer and warmer effectively. And so, the density essentially gets lower and lower as you rise up in the atmosphere. So, when this ash comes out of the volcano, it rises up at very high speeds of 200 metres per second and it mixes with air, low down in the atmosphere and heats up that air. It's that heating of the air lower in the atmosphere that generates this low density mixture of air plus the particles. As we saw before with the bag lifting off the toaster, once we get the density of that air plus the ash to fall below the density of the surrounding air, that mixture can rise into the atmosphere. As it rises, it continues mixing and it'll continue mixing and continue rising until the thermal energy becomes exhausted. And that thermal energy becomes exhausted when it's mixed so much of the cold air in the lower atmosphere that essentially, the bulk temperature of that mixture becomes similar to the much higher temperatures higher than the atmosphere.

Chris - What about the eruption column? How are you going to create that?

Andy - For the volcano, we brought a pump along and we have a tank of red water to denote the red hot ash that is coming out of the volcano. If I turn the pump on, we're going to see the red liquid will come up and the red liquid is fresh water. So, this is a model of very low density water. So, let me turn this on.

Chris - We're now going to blow the water in through a hole which is right in the middle at the bottom of the tank.

Andy - Imagine that Coca-Cola frothing up out of the top of the bottle. It's now coming into the tank and we see this red liquid rising up and you see it's a very turbulent flow. It's mixing and engulfing a lot of the water down here as it rises up. What you see is it's stopped rising at this point and it's now beginning to spread out to the walls of the tank. So now, imagine you're standing down here at some distance away from the volcano and above you, when you look up, you see this red cloud over you - this would be the black ash. And the light from the sun can't penetrate through that big cloud that's spreading out. And so, Pliny would've seen it go progressively darker and darker as that ash spreads out and forms what we call a giant umbrella cloud which will spread out tens of kilometres away from the volcano at that height of about 20-25 kilometres above the ground. The ash will gradually start raining out because the ash is heavy, forming a huge blanket of ash.

Chris - But do you know what sort of mass of ash is going to be ejected by a volcano, sort of like the size of Vesuvius?

Andy - With a big eruption like Vesuvius, we're looking at 106 or 107 kilograms of material coming out per second.

Chris - A thousand tons a second.

Andy - Yes. So, if you think about a domestic fridge, that may be 1 ton. So imagine you got a cubic metre - a fridge - and imagine having 10 million fridges coming out every second. That's the amount of material that you'd see coming out on one of these big eruptions. But of course, it's not the size of a big fridge. There's going to be a whole series of ash particles ranging from very, very small particles that are tens of hundreds of microns which is 1/10 of a millimetre. They go up to the size of this piece of pumice we have here which may be up to about a metre in size. Now, as the eruption at Vesuvius continued, what happened after some time was the pipe or the conduit from the reservoir of magma underground up to the surface became eroded. It was essentially sandblasted by all this fast moving rock going past it. As it eroded, the eruption became faster and faster, more and more intense. Eventually, the very dense mixture that came out of the ground couldn't actually rise up and become buoyant. Instead, it collapsed and formed a flow, a pyroclastic flow. This would run along the ground.

Chris - Can you just explain a little bit about why it does go along the ground again because I didn't quite get that? Why does it not want to carry on going upwards though?

Andy - When it comes out of the volcano, it's by mass, it's mostly the solid material. And so, it's actually quite a lot denser than the air. It's only during the first kilometre or two as it rises that it mixes with enough air and heats that air, so its density falls below the density of the surrounding air. But as the flow rate goes to larger and larger values, it can't mix enough air in that lower kilometre or two to actually become less dense in the surrounding air. So, instead of behaving like a hot air balloon, it's more behaving as if it's a hosepipe pointed upwards with water coming out of it and it goes a certain distance and then it collapses back like a fountain back down to the ground. So, that starts spreading out along the ground. It's still very hot and you still have all these very fine ash. It may be hundreds of meters thick. So, if you imagine tall buildings may be tens of meters high and it's going to be travelling at hundreds of metres per second. You couldn't run away from it and it will be actually very hard to drive away. So, even if you think if Usain Bolt who runs 100 metres in 10 seconds, it will be going 10 times faster than Usain Bolt. So, if we perhaps have a look at the experiment, we can see how this flow works.

Chris - This looks like a fish tank but a little bit longer and a little bit narrower. It's a couple of metres long, 20 centimetres deep, 10 centimetres wide, and it's full of a clear liquid. Is that water?

Andy - So, it's full of salt water and at the end of the tank, at one end of the tank, we have what we call a lock gate. It's a vertical piece which actually separates the first 10 centimetres of the tank from the rest of the tank. Behind that lock gate, I'm going to add some particles to the salty water and so, you can think of these particles as being like the rocks and the ash, mixed up in the flow. Because I add these particles, the density of this fluid behind the lock will become greater than the density of the fluid further down. And so, this is an analogue of looking at the dense flow of rocks and air moving into the air with no rocks in it. So let's pour the particles in and I'll stir it up so I've got a suspension of particles. I'll now pull out the lock gate. What you can see is the flow is running along the tank because it's dense and you see it's a very turbulent structure. It's engulfing lots of the water above it as it runs along. It's gradually slowing down and it's slowing down because the particle load is falling out of the flow. And so, there's nothing to drive it further forward.

Chris - So, what we saw, when we took away the sluice gate was that the mixture went zooming along the floor of the tank which will be like the volcanic cloud coming down the side of a mountain I suppose. But the further away it went, the slower it went and that's eventually because there's no particles left to push it along anymore.

Andy - That's right and that's part of the story about how these very powerful ash flows propagate. But there's a really surprising additional effect that occurs because these ash flows are very hot. And so, I want to show you a second experiment where we're going to include the effect of both particles but also, the fact that the ash flow is hot. Now, the fact that it's hot means that the air in the flow is less dense than the surrounding air. And so, as this flow propagates along and it drops out particles, eventually, it'll have dropped out enough particles that what's left becomes less dense than the surrounding air. At that point, it should lift off the floor and rise up into the atmosphere. I've got a red liquid or put some particles in and this is fresh water now. So, to model it, the fact it's less dense, we're using fresh water. we got salty water in the tank. So, the fresh water alone wants to rise to the top. But because we've put particles in, the mixture of the fresh water in the particles starts off being more dense. It's only when the particles fall out that it can lift off.

Chris - This business about it getting to a certain point and then stopping, are there any sort of contemporary examples because didn't Mount St. Helens do that where you saw trees wiped out, wiped out, and suddenly, you got to a point where all the trees were standing again?

Andy - Yes. So, Mount St. Helens is really the first eruption where this phenomenon was understood because there were very large Douglas fir trees that were very, very big trees and they were all just knocked over like matchsticks by the big pyroclastic flow. But about 15 kilometres from the volcano, there was a line where beyond there, all the Douglas fir trees were still standing. At that point, the flow had mixed with enough air and it dropped out enough particles that the rest of it was less dense and it rose up into the air. And we've now seen a number of examples of this at different volcanoes with flows travelling tens of kilometres and then lifting off and rising into the air.

Chris - Okay, let's give it a go.

Andy - So, what I'm going to do is I'm going to put in my fresh water.

Chris - So what you're doing Andy, you drag the slider back so that we've pushed some of the salt water out of the way and created a space at the end of the tank that you're now filling with the fresh red dyed liquid. And we're now adding some particles in, there you go.

Andy - And then I pull out my sluice gate again. We'll see it runs along the bottom just as before. But as it runs along and drops out the particles, it's now beginning to lift up.

Chris - It suddenly stopped at one point and went straight up in the air. Now, it's all gone to the top.

Andy - So now, it's running along the top surface. You can think of this as being 20 kilometres above the ground and again, it will become very, very dark underneath that cloud. And so, it may be that in a big volcanic eruption, the material doesn't go straight up into the air, but may run along the ground a certain distance before it lifts up. I think now, you can see very nicely all the particles dropping out and this is what Pliny will be describing about - being in a rain of ash particles. Because the lower part of the atmosphere has lots of moisture, all our rain cloud system in it, that moisture is also carried up in this convective structure and it makes the volcanic particles often makes them very wet, and you can get hailstones and rainstorms associated with these big clouds. And so that's why you often see lightning and other effects because of the charge on the particles and also because of the presence of the vapour in the atmosphere. So, they're very dramatic events!